An automotive automatic transmission generally has planetary gears including power transmitting elements such as sun gears and planetary carriers, and a speed change mechanism including hydraulic friction-engaging elements such as hydraulic wet-type multiple disc clutches and hydraulic band brakes. In the automotive automatic transmission, a line pressure created by a hydraulic pump driven by a crankshaft of an engine is used as a drive source for hydraulic friction-engaging elements. The automatic transmission of this type controls the operating conditions of the engaging elements by controlling the oil pressure supply to respective engaging elements, by which the engine torque transmission path on the planetary gear is changed to obtain a desired speed (gear ratio).
An electronic control type automatic transmission uses an electromagnetic oil pressure control valves (hereinafter called solenoid valves) to control the operating conditions of the hydraulic friction-engaging elements. By controlling the on/off or the on/off duty ratio of a solenoid valve, the oil pressure supply to a hydraulic friction-engaging element is allowed or inhibited, or the oil pressure supplied to the friction-engaging element is increased or decreased, by which the friction-engaging element is engaged or disengaged. The hydraulic friction-engaging element to be engaged is changed, that is to say, one of the hydraulic friction-engaging elements relating to speed change is engaged while the other of the hydraulic friction-engaging elements is disengaged, so that a speed change is carried out while the speed change shock in engagement and disengagement of the friction-engaging elements is reduced.
For example, when downshifting is carried out from the fourth speed to the third speed, the hydraulic clutch for establishing the fourth speed (hereinafter called the disengagement-side clutch) is disengaged, and the hydraulic clutch for establishing the third speed (hereinafter called the engagement-side clutch) is engaged simultaneously. By switching the operation of the hydraulic clutches, the engine torque transmission path is changed, so that the downshifting operation is completed.
In the above-described electronic control type automatic transmission, a shift map as shown in FIG. 5, in which the vehicle speed and the throttle valve opening degree are used as parameters, is generally used to select a gear position. From this map, an optimum gear position (target gear position) for the detected value of the vehicle speed and the detected value of the throttle valve opening degree is selected. In the case of kickdown to obtain a quick acceleration, the target gear position is usually determined by the throttle valve opening degree. Specifically, when the throttle valve opening degree crosses a 4-3 shift line or a 3-2 shift line in FIG. 5, a downshifting command is outputted. For example, when the throttle valve opening degree reaches point B from point A in FIG. 5 by the driver's depression of an accelerator pedal, a command for downshifting from the fourth speed to the third speed is outputted. When the throttle valve opening degree reaches point C from point A, a command for a skip downshifting from the fourth speed to the second speed is outputted.
If the driver depresses the accelerator pedal slowly in spite of the fact that the driver has an intention of finally depressing the accelerator pedal to the second speed zone from the fourth speed zone, the shift of the throttle valve opening degree from point A to point C is implemented slowly. In this case, when the throttle valve opening degree crosses the 4-3 shift line (point B), a command for downshifting from the fourth speed to the third speed is outputted. When the throttle valve opening degree crosses the 3-2 shift line, the downshifting from the fourth speed to the third speed may have already been started. In such a case, since the disengagement or engagement of the clutch relating to the downshifting to the third speed is being executed, it is impossible to carry out the skip downshifting to the second speed. Therefore, the downshifting to the second speed must be carried out after the downshifting to the third speed has been completed. As a result, two shift shocks are caused by two downshifting operations, and it takes much time to establish the second speed, thereby degrading the shift response.